In thermodynamics, exothermic and endothermic are used to define the direction or flow of energy in a system. Exothermic or outside heating is the process of releasing energy out of the system. In other words, exothermic reaction produces heat and sometimes can be explosive. Endothermic on the other hand is a process where in a system absorbs energy from the surrounding in the form of heat.

They are forms of energy but they have different direction in relevance to a system so there is a need to differentiate them.

For formal notations, endothermic reaction will have a positive sign or `Delta H > 0` , and exothermic will have a negative sign or `Delta H < 0` .